It is common practice to replace a severely ailing joint with an endoprosthesis. With current commercially available models, surgeons must remove a considerable amount of bone in order to put the implant in place. This compromises bone stock for future implant revision in the advent of implant failure and is one of the major reasons why such operations are rarely performed on young patients. This problem could be alleviated by the use of thin resurfacing implants. Especially when used in highly loaded joints, such implants are subject to bending stresses which can lead to fatigue fracture if they are not properly supported. Ideally, the implant should be placed over hard cortical bone, not cancellous bone as is current practice. This implies that no bone should be removed at the moment of the operation, thereby guaranteeing a healthy bone stock for eventual revision, but at the expense of a high precision customization of the shape of the implant to the particular geometry of the articulating surface of the afflicted joint of each patient.
Since the advent of digital tomographic medical imaging, research teams around the world have strived to generate three-dimensional computer—or numerical—models in order to improve visualization of internal anatomy. Generically, obtaining digitized geometric data from an object and creating a numerical model of said object from the digitized data is often referred to as geometric reverse engineering. Combined with numerically controlled fabrication technologies, whether they be more traditional machines based on removal of material such as numerically controlled (N/C) milling or turning, or on more recent methods based on addition of material on a slice by slice basis—methods known as rapid prototyping, free-form fabrication or other names—it is possible to create a physical, as opposed to numerical, model of internal structures. These physical models can be used for diagnostic purposes or for surgery planning and rehearsal. They can also be used as templates from which a prosthetic element can be fashioned. By manipulating the numerical model, it is also possible to directly fabricate prosthetic elements adapted to the geometry of internal structures.
Conventionally, the method used to reproduce the portion of the body, or for which an implant is to be fabricated, can be described as follows. The body part under investigation is imaged with a medical imaging apparatus. This can be laser or acoustic reflection based apparatus or a number of transmission apparatus such as standard X-ray radiographs, planar or spiral X-ray computer tomography, magnetic resonance imaging, positron emission tomography, magnetic resonance angiography, etc. The images produced are analyzed with readily available image processing techniques such as thresholding, which consists of segmenting or isolating regions on the basis of grey values, mathematical morphology operations such as reduction, expansion, dilatation, etc. and Boolean operations. Once the contours of the desired anatomical structure or structures are identified within each image, a three-dimensional model is generated by interpolating data. This is usually done by using creating a mesh of triangular facets. This method offers many advantages: it is quickly computed, it can be rapidly visualized and manipulated numerically in order to rotate, translate, scale and perform other operations, it is perfectly adapted to the “de facto” standard STL file format used by all rapid prototyping machines. Instead of creating a mesh of triangular facets, one can exploit higher order interpolation functions implemented in CAD systems. These systems can then generate an STL file for fabrication with a rapid prototyping apparatus, with a loss of precision in the process, or G code if fabrication with more traditional material cutting or removal technologies is envisioned.
The method previously described strives to produce as exact a copy as possible of the anatomy under consideration. However, the original data produced by the imaging apparatus is tainted by distortions introduced by the imaging modality. For example, the precision with which the edges of structures can be located is limited by the imaging resolution, or blur, of the apparatus and by imprecision—or noise—introduced by the imaging apparatus. In the methods proposed to this day, knowledge of the distortions has never been exploited in order to improve the original data prior to image analysis. Furthermore, the creation of the three-dimensional model, and the subsequent data generated to drive an N/C fabrication machine can also contribute to further loss of information. Because N/C machines are much more precise than the data obtained by medical imaging modalities, it is common practice to interpolate data. Typically, this consists of interpolating intermediate “slices” between the slices corresponding to the tomographic images. For example, if a rapid prototyping machine can build a layer of 0.25 mm but that the imaging apparatus generates images of structures 1 mm in thickness, then the geometry of three additional layers must be interpolated between two consecutive image layers. The additional layers can be simple repetitions of one of the original image layer, which corresponds to zero order interpolation. A more popular approach, that of generating a triangular mesh and then “slicing” this mesh to the desired machine accuracy, corresponds to first order interpolation. In both situations, discontinuities appear in the model. If one strives for accuracy, higher order interpolation techniques must be used and care must be taken to insure coherent data representation at all stages. It would be useless to use a third degree interpolation scheme, as a NURBS representation implemented in a CAD system for example, if it is later converted into an STL file for fabrication on a rapid prototyping machine, the STL file corresponding to first order interpolation.
When rapid prototyping technologies are used to fabricate the physical model, it is common practice to position the part to be built in the same orientation within the rapid prototyping machine as the patient within the imaging scanner. Therefore, material is added in a slice orientation parallel to the images produced by the imaging apparatus. U.S. Pat. No. 5,741,215 suggests a method for reducing the time, and therefore the cost, required to make a physical model by stereolithography through selective orientation of the model. The author of the patent also claims that the method can be used to fabricate an implant shaped to correct an anatomical defect, implant characterized in having a close fit with connective tissue and contours appropriate for an implant site. Presumably, selective orientation of the model can improve the fit but there is neither mention of improving image data nor of coherent data representation. Furthermore, the proposed application is quite different than having a close fit over the whole surface of the implant in order to insure proper mating with underlying bone.
U.S. Pat. Nos. 5,554,190 and 5,824,083 propose a method based on CAD and image-analysis methods for producing an anchored prosthetic component which provides the largest possible surface for transmission of forces, and its mass and rigidity can be adapted to the individual properties of the bone. Contrarily to resurfacing implants where the loads are principally transmitted perpendicularly to the implant, loads are transmitted parallel to the anchored element, creating very different requirements on design and precision for both applications. Here again, no mention of data improvement nor of data coherency can be found.
In U.S. Pat. No. 5,768,134, the authors set out not only to reproduce the geometry of an anatomical structure but to make a perfected model characterized by at least one artificial functional element with a useful function added to the basic anatomical model. This artificial functional element is created on the basis of the grey value data image information and possibly of additional external information. The external information is provided by the medical user. In order to improve the fit, the authors suggest interpolating contours with sub-pixel accuracy. However, it does not make use of the information on the degradation induced by the imaging modality in order to improve the quality of the images prior to using the image information. Furthermore, as with other proposed methods, there is no particular attention to the three-dimensional representation of the anatomical surface insuring a minimum loss of information.
None of the prior methods or applications addresses the problems posed by the necessity of high precision modeling of critical anatomical surfaces and particularly of methods leading to the fabrication of a mating prosthetic element adapted to the aforementioned anatomical surfaces whereby loads are transmitted in a direction perpendicular to the prosthetic element.